1. Field of the Invention
The invention relates to a developing device for use in an image forming apparatus such as a printer, a facsimile machine and a copier, a process cartridge having the developing device, and an image forming apparatus having the process cartridge.
2. Description of the Related Art
It is generally known in the art that there is an image forming apparatus that employs a hopping developing system. The hopping developing system develops images while toner is hopping on a surface of a toner carrying member. An example of such an image forming apparatus having such a hopping developing system is disclosed in Japanese Patent Application Publication No. 2007-133387. The disclosed image forming apparatus includes a tubular toner carrying member having plural popping electrodes arranged at a predetermined pitch in a peripheral direction of the tubular toner carrying member. In the tubular toner carrying member, A-phase repetitive pulses are applied to the hopping electrodes arranged at even numbered arrangement positions while B-phase repetitive pluses differing from the A-phase repetitive pulses are applied to the hopping electrodes arranged at odd numbered arrangement positions. With this configuration, alternating electric field is generated between the adjacent hopping electrodes to act on the toner to generate electrostatic force such that the generated electrostatic force causes the toner to perform hopping behaviors between the adjacent hopping electrodes. The toner hopping between the adjacent hopping electrodes on the surface of the toner carrying member is then transferred onto a latent image formed on a latent image carrying member to thereby complete the development.
Various experiments have been conducted on the image forming apparatus having the above hopping developing system, and these results have shown that image density, which is generally determined based on a peak value of a pulse voltage, varies with the change in the peak value of the pulse voltage applied to the hopping electrodes. That is, these results have shown that the field intensity near the surface of the latent image carrying member is changed due to the difference in the mean of the pulse voltage when the peak value of the pulse voltage is applied to the hopping electrodes, and the density of the image formed on the latent image carrying member is changed based on the change in the field intensity near the surface of the latent image carrying member.
Note that the change in the peak value of the pulse voltage may result from an environmental change during image forming operation. For example, if the image forming operation is conducted under a high-humidity environment, adhesive force between the toner and the surface of the toner carrying member may be increased due to an increase in liquid cross-linking force, or the electrostatic force generated by the alternating electric field acting on the toner may be decreased due to a decrease in a charging amount of the toner resulting from lowered toner charging efficiency. The above adverse effects inhibit the toner from hopping on the surface of the toner carrying member, causing a decrease in the amount of toner to be transferred on a latent image portion of the image carrying member. As a result, the image density is lowered. If, on the other hand, the image forming operation is conducted under a low-humidity environment, adhesive force between the toner and the surface of the toner carrying member may be reduced due to a decrease in liquid cross-linking force, or the electrostatic force generated by the alternating electric field acting on the toner may be increased due to an increase in a charging amount of the toner resulting from increased toner charging efficiency. In this case, toner is hopping too much to jump up too high on the toner carrying member, which may result in undesired toner adherence to a non-image forming portion of the image carrying member where no electrostatic latent image is formed. That is, toner hopping too high on the surface of the toner carrying member may contaminate the non-image forming portion of the image carrying member (i.e., a base surface of the image forming member).
Further, in addition to the above developing device disclosed in Japanese Patent Application Publication No. 2007-133387, another developing device is also generally known in the art. In the developing device disclosed in Japanese Patent Application Publication No. 2007-133387, hopping toner reciprocates between the adjacent hopping electrodes while the hopping toner is transferred onto a developing region by a surface movement of the toner carrying member. However, in the latter developing device, toner on the surface of the toner carrying member is transferred to the developing region by hopping movements of the toner itself in a predetermined direction. For example, in the developing device having a toner carrying member on which A-phase, B-phase, and C-phase electrodes are repeatedly arranged in this order, toner on the surface of the toner carrying member is caused to perform hopping movements to sequentially transfer the toner on the surface of the toner carrying member from A-phase electrode to B-phase electrode, from B-phase electrode to C-phase electrode, from C-phase electrode to A-phase electrode, and finally to the developing region. However, this type of the developing device may also have similar drawbacks.